1. Field of the Invention
The present invention relates to wireless communication and transmission directivity.
2. Description of the Related Art
For the transmission and reception of data using multiple antennas, a beamforming technique has been employed. Beamforming transmits a single datum via multiple antennas imparting directivity by the multiple antennas by staggering phases on transmission.
FIG. 9 is a schematic showing a conventional wireless communication apparatus. FIG. 9 shows the partial configuration of the wireless communication apparatus on the base station side of a mobile communication system. Data from multiple channels (four channels shown in the figure) is input to a precoder 901, and each channel is given a fixed phase difference to form multiple beams. A transmitting unit 902 includes transmission circuits 902a to 902d for each channel, and transmits data via an antenna 903.
While the configuration shown in FIG. 9 is the configuration of a beamformer that attains a given directivity using the multiple antenna 903, a similar technique that transmits data by imparting phase differences to each channel exists, a multiple input multiple output (MIMO) technique, in which different data is transmitted by each channel (for example, Published Japanese translation of a PCT application No. 2005-522086).
In the configuration shown in FIG. 9, however, there is phase variation among the transmission circuits 902a to 902d for each channel of the transmitting unit 902. The existence of the phase variation causes, at the transmitting unit 902, distortion of the beam formed at the precoder 901, and expected directivity cannot be obtained.
FIG. 10 illustrates gain reduction of multi-beam transmission. As shown on the left side of the figure, under a condition in which there is no phase variation, expected directivity is obtained and transmission can be done with an improved gain (up to 6 decibels (dB)) by the effect of the multi-beam method. Inversely, in the case that phase variation exists in the transmission circuits 902a to 902d of the transmitting unit 902, directivity becomes inadequate, e.g., broaden, and the gains of each beam is reduced (down to 4 dB).
Therefore, in order to eliminate phase variation, phase adjustment has formerly been required so that phase variation does not occur by comparing the outputs of the transmission circuits 902a to 902d of the transmitting unit 902 and a reference signal using an external calibrator.